Ankle ligament injuries are extremely common, accounting for up to 28% of all athletic injuries. However, little is known about risk factors for these injuries or for susceptibility to repeated injuries of this kind. In previous studies, a repeatability study established the ability to reliably measure the factors that have been implicated in predisposing an athlete to ankle injury. The purpose of this four year study is to build upon these earlier studies and perform a comprehensive investigation of ankle injury risk factors. Approximately 666 pre-collegiate or collegiate athletes with no history of significant ankle trauma and who use no bracing or taping will be studied prospectively. These include males and females participating in soccer, lacrosse and field hockey, as well as in basketball and tennis. Evaluation will be based on potential risk factors suggested by preliminary and other studies, demographic information, morphometry, ankle joint laxity, anatomic alignment of the foot and ankle, isokinetic ankle strength, postural stability, and muscle reaction time in response to sagittal and frontal plane perturbations of the foot. Each athlete will then be followed in order to document their exposure to practices and games. Using these data, one or a combination of intrinsic risk factors will be used to predict which individuals are at increased risk for ankle ligament trauma and to characterize gender differences in the effects of intrinsic risk factors.